Presidents who want to shape the world unilaterally must face four inconvenient legal truths.

President Donald Trump’s announcement that he will consider using an executive order to get a citizenship question on the 2020 census is but the latest presidential attempt to exaggerate or mythologize the power of the executive order. But a president who wants to shape the world via executive order must face four inconvenient legal truths.

The first is that the Constitution gives presidents very little peacetime domestic power to do anything that affects the world beyond the executive branch itself. Regulating interstate and foreign commerce—that’s a congressional power. Raising and supporting the military—likewise. So are a great many other things, such as the control of public lands, the protection of intellectual property—and the direction of the census.

The second truth, implied by the first, is that before a peacetime president may affect the public at large, he must find a license to do so in a statute that Congress has enacted. Congress exercises its powers through statutes, hiring the executive branch to serve as its agent. To combat pollution, it authorizes the Environmental Protection Agency. To administer federal disability insurance, it vests power in the Social Security Administration. And so on. Congress sometimes grants administrative authority directly to the president, but that’s the exception, not the rule. (One important exception is the grant of authority to exclude foreign nationals from the U.S. on national-security grounds, which is the authority on which Trump based his travel-ban orders.)

The third is that every statute that authorizes the executive branch to do something also puts limits on that power. Lawful executive action must respect those limits, what I call in my administrative-law classes an administrator’s “zone of discretion.” For example, the Census Act requires the secretary of commerce to submit, at least three years before the census date, a report containing the secretary’s “determination of the subjects proposed to be included in the census, and the types of information to be compiled.” The secretary must then submit, at least two years before the census date, a report containing the secretary’s “determination of the questions proposed to be included” in the census. Should the secretary of commerce decide just a year before the census date to ask each person his or her favorite Beatles song, the question would be impermissible because it was not reported to Congress in time.

Finally—and this is the speed bump over which the Commerce Department stumbled—under the federal Administrative Procedure Act, responsible officials must also, at the time a decision is made, offer a nonarbitrary, non-capricious explanation for choosing their strategy for carrying out their statutory authority. This requires at least that the proposed administrative action be a rational way of carrying out the mission Congress has assigned the executive, and that the rationale be clearly explained at the time of an agency decision. In the case of the citizenship question, the Department of Commerce failed to take this step, which is why a 2020 citizenship question is now on hold. Even if such a question could have been asked, in principle, consistent with the secretary’s zone of discretion, the failure of Commerce Secretary Wilbur Ross to offer a non-pretextual rationale for the citizenship question tripped him up in the courts. As euphemistically summarized by Chief Justice John Roberts for a Supreme Court majority, the secretary offered a “contrived … explanation for agency action that is incongruent with what the record reveals about the agency’s priorities and decision making process.”

The executive branch now has an obvious logic problem: It must generate a non-pretextual agency explanation to justify an action to which it already committed itself without any such explanation. The Justice Department’s public flailing-about for a new rationale looks like the very definition of “arbitrary and capricious.” But whether or not such a rationale can now be found and belatedly attached to the census question, a presidential order cannot make the job easier.

For one thing, a presidential order cannot expand the secretary’s zone of legal discretion to determine the contents of the census. Just as important, an executive order cannot relieve the commerce secretary of his obligation to proceed based on a nonarbitrary rationale rooted in his statutory mission. “The president wants me to ask this question” would not be an adequate reason. Nothing in the statute empowers presidential whim.

Among administrative-law scholars, a serious debate exists over whether the Constitution authorizes the president to command how subordinate officials exercise their discretion. There is no debate, however, as to whether a president may command administrative action that Congress has not authorized. He may not. The Supreme Court stated the principle in 1838: When Congress “impose[s] upon any executive officer any duty [Congress] may think proper which is not repugnant to any rights secured and protected by the Constitution … in such cases, the duty and responsibility grow out of and are subject to the control of the law, and not to the direction of the President.”

Trump is not the first president to exaggerate the power of executive orders, in word if not attempted deed. Following the 2012 shootings at Sandy Hook Elementary School, President Barack Obama characterized a series of unilateral initiatives, including an executive order and two presidential memorandums, as his direct responses to congressional inaction on guns. But his orders did no more than tell various administrative agencies to work faster, more efficiently, and with greater coordination, and to conduct additional research aimed at curbing gun violence. Without a statute, he could not actually require the public to do anything.

Trump has taken a page from Obama, doing all he can to obscure the limited reach of his executive orders. Whether parading his orders as the undoing of Obamacare, or the rebirth of free speech on campus, he has most often done nothing other than tell subordinate officials to study, report, and consider how to carry out their legal discretion in a manner consistent with the president’s policies. In the case of his sanctuary-cities order, in which he arguably went too far by threatening to withhold federal grants to those cities, the Ninth Circuit rebuffed him: “Under the principle of Separation of Powers and in consideration of the Spending Clause, which vests exclusive power to Congress to impose conditions on federal grants, the Executive Branch may not refuse to disperse the federal grants in question without congressional authorization.”

The spectacle of the executive order fits the president’s yearning to appear the ever-muscular leader overleaping bureaucratic niceties on the way to policy triumphs. But law can trump myth. The secretary of commerce can satisfy the “arbitrary and capricious” test only with non-pretextual reasoning to support a citizenship question as a rational strategy for making the census useful. Any order purporting to relieve him of that obligation will simply expose the president’s weakness.